Scientists Find Bacteria Using Sunlight to Increase Infections

Researchers working at the University of California, Santa Cruz, Carnegie Mellon University and Stanford University have discovered that exposure to sunlight can cause a 10-fold increase in a virulent bacteria's growth and its resistance to the body's natural immune system. This is the first time a relationship between bacterial disease and sunlight has been found.

A team of scientists discovered that the organism Brucella which causes the disease brucellosis (also known as Malta fever) in cattle and humans contains several genes in common with green plants. The genes cause plants to react to blue wavelengths of light and steer their growth toward sunlight.

Co-author, Winslow Briggs at the University of California, Santa Cruz, first discovered genes for the plant phototropins about ten years ago. In a statement released by the National Science Foundation, the lead author, Trevor Swartz also at UCSC, says "Brucella bacteria have been very well studied for years, and no one knew they could sense light. And now it seems like it's a common thing rather than being an anomaly."

Scientists cultured the bacteria with a supply of macrophages, the body's first line of defense against bacteria, which literally engulf and consume an invading organism. When cultured together in the dark, normal growth and destruction of the Brucella strain was seen. But when exposed to light, the bacterial growth ballooned, literally reproducing inside the macrophage fast enough to make it burst.

Briggs was at first surprised by the find of the genes in Brucella. Since their discovery, he hypothesizes that Brucella uses the genes to increase its virulence. In a statement to the Stanford News Service, Briggs said "Put a flashlight up to your hand-a lot of light shines through. My hypothesis is that as macrophages get close to the skin they get enough light to activate some of these proteins. That pumps up the bacteria's virulence, and my guess is that it does that by turning on defense machinery in the bacteria, to defend itself against the macrophages."

The paper's other authors are Tong-Tseung Tseng of the Carnegie Institution; Jung-Gun Kim of the Department of Biology at Stanford; Marcus Frederickson of UC-Santa Cruz; Diego Comerci and Rodolfo Ugalde, both of the Universidad Nacional de San Martín (Argentina); and Gireesh Rajashekara and Gary A. Splitter, both of the University of Wisconsin-Madison. The paper appeared in the journal Science.

Human cases of brucellosis are fairly rare in the United States, but an estimated 500,000 people, mainly in developing countries, contract the disease annually. In cattle the disease can be fatal and cause spontaneous abortions. In humans, the disease can cause debilitating flu-like systems and muscle aches that can last for weeks or even months. It can also cause meningitis and even encephalitis, although it is rarely fatal.

So far, scientists have discovered about 12% of the bacterial species that have had their genetic material sequenced contain the LOV protein domain that causes the bacteria to produce the light-sensitive proteins. In plants, the proteins work by changing shape when hit by sunlight. How it causes bacteria to change their growth still remains a mystery.

"How does this mechanism work in the cell? This protein gets activated-does it activate another protein? Then how does it increase infection?" Swartz asks. "And why is it important for the bacteria to have this function? The work we've done is just the tip of the iceberg."